In a television camera, respective levels of image signals for R, G and B channels corresponding to a white subject are adjusted to the same level, thereby keeping a white balance. This causes the white color to be reproduced to a color in which the coloring is not lost and is not different from that viewed by naked eyes, thereby improving the color reproducibility of the whole screen, even if a light radiating the subject is the sun light or an artificial light.
The correction of the white balance in conventional television cameras have been performed in the following manner. That is, a gain is adjusted so that the G signal level becomes the same level as the R signal and the B signal levels by setting the diaphragm value at value at which the optical properties of the taking lens become stable, for example, f 8.0, taking an image of a white subject on the whole screen, and performing the white balance correction.
Now, taking lenses of recent television cameras have been improved in the optical properties, and the signal processing technology has been progressed, so that most of them have the opening diaphragm value of about f 1.4, whereby they can take a subject even at a dark place because the range of the diaphragm spreads to opening side.
In cameras having the taking lens with such opening diaphragm value of about f 1.4, the white balance is successfully kept in a condition in which the diaphragm is closed in a manner that the diaphragm value exhibits a value, for example, more than f 2.8.
However, when an image is taken by opening the diaphragm near to the limit at which the diaphragm value exhibits a value less than f 2.8 in a place short of illumination such as in door, even though the white balance adjustment has been performed, the whole screen may be short of green color, or surplus of red color, so that the image may be colored slightly with magenta color (for short of green color) or with red color (for surplus of red color) to cause the color reproducibility to be decreased.
Hence, current situation is such that there has been made a selection of either one method in which an image is taken at a larger diaphragm value by making up illumination at image taker side in a place short of illumination, or that in which an image is taken by leaving the image colored slightly with magenta color or red color as it is.
Thus, the fact that, in image taking in a condition in which the diaphragm is opened neat to opening, the screen assumes magenta color or red color is because the trend of an increase in R, G and B signal levels associated with opening of the diaphragm becomes different near the diaphragm opening end so that G signal or R signal level becomes relatively lower (G signal) or becomes relatively higher (R signal) than other signal levels.
That is, as shown in the characteristic graph of FIG. 3, when opening the diaphragm near to opening, associated with it, the image signal level of the R, G and B channels obtained through a solid image sensor device (commonly known as CCD) corresponding to a three-color separation optical system and to respective colors increases, while in a region where the diaphragm is relatively closed, the trend of an increase in the R, G and B signals (each of which is an image signal) is uniform.
On the other hand, in a region where the diaphragm is largely opened and the diaphragm value exhibits f 2.8 or less, the increasing trend varies among the R, G and B signals such that for the G signal, its level becomes lower than that of other color signals, and for the R signal, its level higher than that of other color signals.
It is assumed that such characteristics are due to the way of how to deploy the light separated by a prism (where the G signal decreases) or to a variation in the light collecting characteristics of colors in respective solid image sensor device (where the R signal increases), and such characteristics are unique to television cameras using the solid image sensor device, or to three-tube, three-plate type cameras having a color separation optical system such as prism.
In conventional white balance correcting method, taking a slightly closed diaphragm value such as f 8.0 as a reference, by level adjusting the R, G and B signals under such certain diaphragm value, the white balance for the whole region of diaphragm is kept uniformly, thereby providing a method neglecting the decrease of the G signal level or the increase of the R signal level at the diaphragm opening end.
Hence, thereafter taking an image by opening the diaphragm near to the limit causes the G signal or the R signal to tend to be decreased or increased with respect to other signal levels, in spite of the white balance adjustment in advance, whereby the green color is short, or the red color is surplus, so that the image assumes magenta color or red color.
Therefore, it is an object of the present invention is to provide a television camera and a white balance correcting method wherein even in a condition in which the diaphragm is opened to the limit, the white balance is properly kept so that even in a place having a small quantity of light, an image having a good color reproducibility is obtained.